Satellites have complex systems with various analog and digital circuits and components that are interconnected through a complex wiring schematic. Commonly, the various systems within a satellite are tested prior to launch.
The functioning and interconnections between the systems are checked on a system operability level. The systems are not necessarily checked during assembly or incorporation into the satellites or their functions with other systems. Also, since satellites have back-up systems, the systems may default to their back-up modes of operation without alerting ground control of an error. The systems may operate for months or years without detection of the activation of the back-up mode.
It is desirable to test the operability of various circuits while the satellite is in orbit and determine their status and functionality. Further, it is desirable to test the operability of the circuits while the satellite is being assembled.